1. Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus for performing the heat treatment of a thin film which is formed on a base material including a semiconductor wafer, a glass substrate for a liquid crystal display device and the like and which is made of material different from that of the base material.
2. Description of the Background Art
In a manufacturing process of a semiconductor device, introduction of impurities is an indispensable process for forming a PN junction in a semiconductor wafer. Now, the introduction of impurities is generally conducted by an ion implantation method and a subsequent annealing method. The ion implantation method is a technique in which ionized impurity elements such as boron (B), arsenic (As), and phosphorus (P) are physically injected by colliding them with a semiconductor wafer with a high acceleration voltage. Injected impurities are activated by an annealing treatment. In this case, when an annealing time is about several seconds or more, the injected impurities are deeply diffused through heat, and consequently there is a possibility that a junction depth becomes much deeper than a required value, which may cause trouble in forming a good device.
Accordingly, flash lamp annealing (FLA) has attracted attention in recent years as an annealing technique for heating a semiconductor wafer for quite a short time. The flash lamp annealing is a heat treatment technique in which only the surface of a semiconductor wafer, where impurities are injected, is heated up for quite a short time (several milliseconds or less), by irradiating the surface of the semiconductor wafer with flash light using a xenon flash lamp (hereinafter, just a “flash lamp” refers to a xenon flash lamp).
The radiation spectral distribution of the xenon flash lamp ranges from an ultraviolet region to a near-infrared region, and its wavelengths are shorter than those of a conventional halogen lamp and are mostly in agreement with the fundamental absorption band of a silicon semiconductor wafer. Therefore, when being irradiated with flash light from the xenon flash lamp, the semiconductor wafer can be quickly heated up because the amount of transmitted light is small. Furthermore, it has been proven that only the vicinity of the semiconductor's surface can be selectively heated up when flash light irradiation is performed for quite a short time of about several milliseconds. For this reason, when temperature is raised in an ultra-short time by the xenon flash lamp, only impurities can be activated without diffusing the impurities deeply.
As a heat treatment apparatus using such a xenon flash lamp, US2006/0291835 discloses an apparatus in which a semiconductor wafer is placed on a hot plate to be preheated to a predetermined temperature and then is heated up to a desired treatment temperature by flash light irradiation from the flash lamp. Furthermore, a heat treatment of a film, which is formed on the surface of a semiconductor wafer, by using flash light irradiation is attempted, and Japanese Patent Application Laid-Open No. 2001-332484 discloses that edge roughness is improved by performing flash light irradiation on a resist film after a development processing procedure.
In the art disclosed in Japanese Patent Application Laid-Open No. 2001-332484, the resist film is formed on a silicon semiconductor wafer, and a flash heat treatment is performed for the resist film. Furthermore, an amorphous silicon (non-crystalline silicon) or poly-silicon (polycrystalline silicon) thin film is formed on a silicon dioxide (SiO2) base material, and the flash heat treatment of the thin film is attempted to promote crystallization.
Thus, when a flash heat treatment is performed for a thin film which is formed on a base material and is made of material different from that of the base material, particularly when the flash heat treatment is performed for a silicon thin film formed on a base material such as silicon dioxide with relatively small thermal conductivity, thermal conduction is hardly produced from the thin film to the base material. For this reason, even if flash light irradiation is performed for quite a short time of about several milliseconds, exfoliation of a film or thermal damage in the film itself may arise because the film is excessively heated.